I. Field of the Invention
The apparatus of this invention is an antenna system especially useful in a mobile, airborne, or satellite communication system, or in any other application in which multiple-beam, multiple frequency band microwave transmission and reception are required in an especially compact package. However, it is principally intended for use in satellite communication stations for use both in military and civilian systems.
Such satellite communication systems have come to be used for a variety of purposes such as meteorological data gathering, ground surveillance, the telemetry of various other data, and the retransmission of commercial television entertainment programs. Since the cost of placing a satellite in orbit is considerable, each satellite must desirably serve as many communication purposes and cover as many frequency bands as possible. In order to serve many of the purposes for which satellites are useful, the communication system must be able to accurately "tailor" the transmission and reception patterns or Earth in such a way as to accurately control the regions to which transmissions are being directed, and from which signals are being received. Moreover, the accurate control of signal strength within a given transmission area makes possible the production of greater signal strength in precisely those local regions within the transmission area where reception would otherwise be difficult because of geography or jamming of the signal by other signal sources, etc. For these reasons, the use of multiple-beam transmission has come into being, permitting many widely separated areas to be placed into communication with one another, or permitting the accurate shaping of the beam profile to fit the reception or transmission area.
The corresponding necessity to generate a composite beam from a number of discrete beamlets has led to the substitution of multiple feed arrays in place of the single horns formerly used to feed the antenna system. Such arrays consist simply of a relatively large number of horns grouped closely together and individually energized. Their radiations are directed toward a common reflector which produces the focussed beam or beams propagating toward Earth. Since such multiple feed arrays may comprise as many as one hundred individual horns, their size is large enough to render their placement directly on the reflector axis impractical from the standpoint of excessive beam interception by the feed array itself. Consequently, offset designs in which the feed array is placed slightly to one side of the main propagating beam from the reflector antenna have come into use.
Such offset designs have been quite well developed, and have resulted in very successful multiple-beam antenna systems. However, each multiple feed array has generally required its own reflector antenna, a requirement which quickly becomes onerous in the case that several different frequency bands, each needing a separate feed array, have to be accommodated. Reflector antennas and the space needed to store them on a satellite prior to deployment are major items of bulk and weight in the total of all equipment used in a satellite communication station. Consequently, great advantages in terms of reduced cost and complexity would result if it were possible to use but a single reflector antenna for all of the multiple-beam feed arrays, and hence for all frequency bands for which the satellite is intended. Conversely, the number of communication channels per satellite could be expanded, such that the total cost of providing and maintaining a large range of satellite communication services could be reduced.
II. Description of the Prior Art
U.S. Pat. No. 3,148,370 issued Sept. 8, 1964 to D. F. Bowman, covering a particular pattern of a frequency-selective mesh for use as a microwave reflector. However, the patent does disclose at FIGS. 7 & 8 and the corresponding portions of the specification, an antenna system which may be used to form a single beam from two microwave feed sources. However, the feed sources utilized by Bowman are single horns, one of which must actually be mounted in the main reflector of his Cassegrainian reflector system, while the other is located at the prime focus of the reflector. Consequently, there is no way to expand the Bowman system to accommodate a plurality of microwave feeds operating at several or many different frequencies. Moreover, this prior art antenna system really is not appropriate for use with multiple-beam feed arrays in any case, since such arrays are generally so large as to make their use inefficient unless they are located off the principal axis of the optical system, as stated in section I. of this disclosure.
U.S. Pat. No. 3,394,378 issued July 23, 1968 to LaVergne E. Williams et al, and disclosed a Cassegrainian-Gregorian antenna system (FIG. 3) in which the microwave radiations of three separate feeds are combined into a single beam by the use of a single frequency-selective surface 25. Once again, the feeds involved are merely single horns, and the antenna system is an entirely rotationally symmetrical type which would not be appropriate for use with large multi-beam feed arrays because of the aforementioned problem of beam cutoff and interception caused by the presence of the feed arrays themselves.
U.S. Pat. No. 3,271,771 issued Sept. 6, 1966 to P. W. Hannan et al, and disclosed an antenna system based on the Cassegrainian model in which a first microwave source located at the Cassegrain focus, and a second source at the prime focus are combined in the output beam by means of a polarization-sensitive secondary mirror. Except for the teaching of the use of discrimination between the two sources on the basis of polarization, the Hannan et al patent adds nothing of significance to the prior art under discussion here. In particular, Hannan et al does not address the problems of how to accommodate a multiple feed array in his antenna system, or how to extend his system to encompass the use of more than two feeds.
U.S. Pat. No. 4,017,865 issued Apr. 12, 1977 to Oakley McDonald Woodward, and covers a Cassegrainian system very much like the one utilized by Hannan and others, excepting the use of a frequency-selective surface instead of a polarization-sensitive surface as the subreflector.
U.S. Pat. No. 3,281,850 to P. W. Hannan issued Oct. 25, 1966, and details a number of antenna systems in which single or dual microwave feeds are combined in Cassegrainian systems using frequency differences in the sources as a basis for subreflector discrimination. Again, there is no suggestion as to how to extend the system to three or more sources, or as to how to accommodate multiple feed arrays in this axially symmetric system.
U.S. Pat. No. 3,231,892 issued Jan. 25, 1966 to J. L. Matson et al, and covers a Cassegrainian system in which two sources at different frequencies are combined in the output beam by means of a frequency-selective subreflector.
U.S. Pat. No. 3,769,623 issued to Fletcher et al. on Oct. 30, 1973, and covers a particular design of dichroic plate in which the frequency stability with changes in incident angle is claimed to be improved. However, the patent also discloses a Cassegrainian antenna system utilizing the plate as a frequency-selective element to place an X-band and S-band source at the Cassegrainian focus of the system. However, the system is entirely unsuitable for use with relatively large multiple feed arrays, since both microwave feeds are placed within the region of space between the main and secondary reflectors of the Cassegrainian system.